Every public pronouncement they've made about it. The Krugman piece linked elsewhere in these comments examines a few in more detail if you don't find them instantly indicative of non-seriousness without further context (but you should).
Vague generalizations mean nothing.
Also here's another prediction from the esteemable Krugman:-
"The growth of the Internet will slow drastically, as the flaw in ‘Metcalfe’s law’—which states that the number of potential connections in a network is proportional to the square of the number of participants—becomes apparent: most people have nothing to say to each other! By 2005 or so, it will become clear that the Internet’s impact on the economy has been no greater than the fax machine’s.”
